Overview
An open circuit fault on an Apollo XP95 addressable loop is one of the most common fault types you'll attend. It means the panel has detected a break in the loop wiring. On a loop wired out from the panel and back again (the normal UK arrangement), a single break usually leaves every device still communicating, because the panel can reach each side of the break from its own end of the loop; it reports the open circuit rather than losing devices. If devices are actually missing as well, suspect a second break, or a section wired as a spur rather than as part of the loop.
This guide walks you through diagnosing and clearing an XP95 open circuit fault systematically, using the information the panel gives you and a logical approach to cable tracing.
What the Panel Is Telling You
XP95 is Apollo's device range, so the exact wording comes from the panel make and model, not from Apollo; check that panel's manual. Typically you will see:
- Fault type: an open-circuit (or "loop open") fault
- Loop number: which loop is affected
- Any devices not responding: if there are some, note their addresses (see above for what that suggests)
Write these down before you change anything. Some panels can also tell you which devices they reach from each end of the loop, which locates the break directly; the panel manual will say whether yours can.
Probable Causes
Open circuit faults on XP95 loops are usually caused by one of the following, roughly in order of likelihood:
1. Cable break between devices — Physical damage to the loop cable between the last online device and the first offline device. This is the most common cause. Cable can be damaged during other works, pinched under cable trays, or suffer insulation failure over time.
2. Faulty or improperly seated device base — A detector base that isn't making proper contact can interrupt the loop continuity. Even a single faulty base with poor pin contact can break the circuit for everything downstream.
3. Loose or corroded termination in a junction box — Wago connectors, traditional terminal blocks, and Choc blocks in field junction boxes can work loose, corrode, or have single-core cable pulled out slightly. This is especially common in damp environments or where cable trays experience vibration.
4. Failed isolator module — XP95 systems use isolator modules at strategic points in the loop to section off faults. If an isolator module has failed in the tripped position, it will appear as an open circuit beyond that point. Unlike a genuine cable break, the isolator module LEDs may give you additional information.
5. Device removed from base — Someone has removed a detector from its base without replacing it, and the base hasn't been fitted with a loop-continuity shorting plug. This isn't a failure — but it presents identically to an open circuit.
Diagnostic Procedure
Step 1: Record the fault information
At the panel, note the loop number and any devices that are not responding. To find where the break is on an intact-but-open loop, a common approach (with the Responsible Person informed and the work covered by your safe system of work) is to disconnect one end of the loop at the panel: the devices that then stop responding are the ones beyond the break, and the boundary between responding and non-responding devices marks it. Say that boundary falls between address 42 and address 43. Reconnect the loop end afterwards.
Step 2: Identify the physical location of the boundary devices
Using the system's as-installed drawings, find the physical location of the last responding device (address 42 in our example) and the first non-responding one (address 43). Addresses are not necessarily in cable order, so go by the drawings, not the numbers. The fault is somewhere in the cable run between them — or at address 42's base, or address 43's base.
Step 3: Check the obvious culprits first
Before tracing cable, do a quick visual check:
- Is there any device obviously missing from a base in the affected area?
- Are there any junction boxes that look recently disturbed?
- Has any other trade been working in the area?
Step 4: Check the boundary device bases
Remove the detector from address 42's base and check:
- Are the loop cable cores properly terminated in the base?
- Is the screw terminal tight?
- Are there any signs of overheating or arcing?
Then check address 43's base the same way. Reinstall the detectors after checking — a detector that isn't seated properly can cause intermittent loop issues.
Step 5: Trace the cable run
With a multimeter set to continuity, you can check the cable between the two boundary devices:
- Disconnect the loop cable at address 42's base (outgoing side)
- Short the two conductors together at address 43's base (or the far end of the run if the base isn't accessible)
- Check for continuity back at address 42's outgoing terminals
No continuity confirms a cable break. Now walk the route: look for damage, pinch points, or any area where cable might have been disturbed.
Step 6: Check junction boxes in the run
Every junction box between addresses 42 and 43 is a possible fault point. Open each one and check that all terminations are tight and that conductors aren't corroded or damaged.
Step 7: Check for isolator modules
If your system uses isolator modules, identify any that fall between addresses 42 and 43. An isolator module that has operated (tripped) due to a downstream short circuit will appear as an open circuit once the original short is cleared. Check the isolator module's status LEDs.
Resolution
Once you've located the break:
- Cable break: Depending on severity — repair, splice, or replace the cable run. Ensure any joints are made in accessible junction boxes, properly terminated, and documented.
- Loose termination: Re-terminate the conductor properly. On older wiring, it's worth checking adjacent terminations in the same box while you're there.
- Failed isolator: Replace the isolator module.
- Missing detector: Fit a replacement detector or a proper continuity plug in the base.
After resolving the fault, restore the panel from fault condition and confirm all previously offline devices come back online.
Standards Reference
Open circuit faults on addressable loops should be investigated and resolved in accordance with:
- BS 5839-1 — particularly clauses relating to fault investigation, repairs to cables, and documentation of remedial works
- The Apollo XP95 Technical Manual — for loop impedance limits, isolator placement requirements, and device addressing guidance
Always document the fault, the probable cause identified, the resolution carried out, and the date. This forms part of the fire alarm system's service record.
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